linux-kernel - [PATCH v2 6/7] mm: thp: Use more portable PMD clearing sequenece in zap_huge

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Message-Id: <20121004.154747.650416442352280556.davem@davemloft.net>
Date:	Thu, 04 Oct 2012 15:47:47 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	linux-mm@...ck.org
CC:	sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, akpm@...ux-foundation.org,
	aarcange@...hat.com, hannes@...xchg.org
Subject: [PATCH v2 6/7] mm: thp: Use more portable PMD clearing sequenece
 in zap_huge_pmd().


Invalidation sequences are handled in various ways on various
architectures.

One way, which sparc64 uses, is to let the set_*_at() functions
accumulate pending flushes into a per-cpu array.  Then the
flush_tlb_range() et al. calls process the pending TLB flushes.

In this regime, the __tlb_remove_*tlb_entry() implementations are
essentially NOPs.

The canonical PTE zap in mm/memory.c is:

			ptent = ptep_get_and_clear_full(mm, addr, pte,
							tlb->fullmm);
			tlb_remove_tlb_entry(tlb, pte, addr);

With a subsequent tlb_flush_mmu() if needed.

Mirror this in the THP PMD zapping using:

		orig_pmd = pmdp_get_and_clear(tlb->mm, addr, pmd);
		page = pmd_page(orig_pmd);
		tlb_remove_pmd_tlb_entry(tlb, pmd, addr);

And we properly accomodate TLB flush mechanims like the one described
above.

Signed-off-by: David S. Miller <davem@...emloft.net>
---
 mm/huge_memory.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 20eeb2b..2fa0c59 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -996,9 +996,10 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	if (__pmd_trans_huge_lock(pmd, vma) == 1) {
 		struct page *page;
 		pgtable_t pgtable;
+		pmd_t orig_pmd;
 		pgtable = pgtable_trans_huge_withdraw(tlb->mm);
-		page = pmd_page(*pmd);
-		pmd_clear(pmd);
+		orig_pmd = pmdp_get_and_clear(tlb->mm, addr, pmd);
+		page = pmd_page(orig_pmd);
 		tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
 		page_remove_rmap(page);
 		VM_BUG_ON(page_mapcount(page) < 0);
-- 
1.7.10.4

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